1. Technical Field
The present disclosure relates to a low consumption voltage regulator and to a voltage regulation method, in particular for regulating the output voltage of a high voltage charge pump and to a memory device provided with the voltage regulator.
2. Description of the Related Art
As is known, charge pumps are typically used in circuits that require internal operating voltages higher than the supply voltage, as, for example, in the case of non-volatile EPROM and FLASH memories, where the programming and erasure voltages are of approximately 12-14 V, whilst the supply voltage is of approximately 1-3 V. Operation of a charge pump is based upon the transfer of charge from a supply pin (biased at the supply voltage) to an output stage of the charge pump, including an accumulation capacitor configured for accumulating the transferred charge, consequently increasing the value of an output voltage that is set up across it. The output voltage depends upon the charge transferred. In order to regulate automatically the amount of current supplied to the accumulation capacitor to maintain the voltage across it constant, it is common to use a voltage regulator circuit configured for detecting the voltage at output from the charge pump (or a voltage proportional to the output voltage) and controlling in feedback the current supplied to the accumulation capacitor in order to maintain the output voltage at a desired value, substantially constant.
Operation of a regulator circuit generally envisages comparison of a division of the output voltage (or of a respective output current) with a reference voltage (or with a respective reference current), which represents the voltage (or current) that it is desired to obtain. The result of the comparison is a state signal, which indicates that a desired level of output voltage has been reached and is configured for acting on the charge pump for interrupting transfer of charge towards the accumulation capacitor.
However, for low supply voltages, generally between approximately 1 V and 1.4 V, the charge pumps lose efficiency, reaching values of inefficiency factor Ifactor of 60 or higher. The inefficiency factor Ifactor is given by the ratio between the current required of the supply by the charge pump and the current delivered by the charge pump. The current required by the regulator circuit (which in effect represents part of the load of the charge pump) to the charge pump results in a corresponding current required of the supply by the charge pump multiplied by the inefficiency factor Ifactor.
Limiting the current consumption of the regulator circuit is consequently of fundamental importance.
FIG. 1 shows a resistive-divider regulator circuit 1, of a known type, configured for generating at output an enabling signal for the transfer of charge VON/OFF, and connected to a charge pump 2, which is also of a known type, designed to receive on an input terminal 2a the signal for enabling charge transfer VON/OFF and generating on an output terminal 2b a charge signal VOUT. The regulator circuit 1 includes a comparator 4 configured for receiving on a first input 4a a reference signal VREF and on a second input 4b a comparison signal VP, given by a division of the charge signal VOUT, and for generating at output the signal for enabling charge transfer VON/OFF on the basis of the comparison between the comparison signal VP and the reference signal VREF. The regulator circuit 1 moreover includes a first reference resistor 10, connected between the output terminal 2b of the charge pump 2 and the second input 4b of the comparator 4, and a second reference resistor 12, having an adjustable resistance value, connected between the second input 4b of the comparator 4 and a ground terminal GND. The first and second reference resistors 10 and 12 hence form a resistive divider of the charge signal VOUT. In particular, the comparison signal VP is the division of the charge signal VOUT taken on the second reference resistor 12. The charge signal VOUT of FIG. 1 is a voltage signal, and its desired value is given by the following formula (1):VOUT−VREF·(1+RP/RR)  (1)where RP is the resistance value of the first reference resistor 10 and RR is the resistance value of the second reference resistor 12.
The regulator circuit 1 of FIG. 1 presents some disadvantages, above all in the case where it is desired to maintain low levels of consumption (for example, consumption levels of approximately 1 μA). In fact, for the purpose it would be necessary to increase the value of the resistances RP and RR, for example to values equal to or higher than 10 MΩ. However, this is not always possible, since the resistance values would require a considerable occupation of area, which is scarcely available in circuits of an integrated type.
FIG. 2 shows a regulator circuit 20 of a known type, alternative to the regulator circuit 1 of FIG. 1, and in particular affording the advantage of requiring low consumption levels irrespective of the area occupied.
The regulator circuit 20 of FIG. 2 drives a charge pump 2 by means of a signal for enabling transfer of charge VON/OFF, as has already been described with reference to FIG. 1. However, in this case, the charge signal VOUT is a voltage designed to bias a reference branch 22 of the regulator circuit 20. The reference branch 22 includes a plurality of Zener diodes 24 (three Zener diodes 24 are illustrated in the figure), which are biased by a reference current IREF. The reference branch 22 is connected to a ground terminal GND via a first mirror transistor 25, having diode configuration, i.e., with the control terminal connected to its own source terminal. Furthermore, the control terminal of the mirror transistor 25 is connected to the ground terminal GND via a Zener diode 26.
The reference current IREF is generated by means of a current generator 27 having a first terminal connected to a supply voltage VDD and a second terminal connected to a conduction terminal of a second mirror transistor 28. The first and second mirror transistors 25 and 28 are moreover connected to one another in current mirror configuration 30. Furthermore, the second terminal of the current generator 27 is connected to an inverter 29, which generates at its output the signal for enabling transfer of charge VON/OFF supplied at an input 2a to the charge pump 2. In use, when the charge voltage VOUT does not exceed a regulation voltage value (desired voltage), the reference branch 22 is traversed by a current having a value equal to the reference current IREF. The signal for enabling charge transfer VON/OFF generated at output by the inverter 29 governs the charge pump 2 in normal operating conditions (i.e., the charge voltage VOUT on the output 2b of the charge pump 2 increases). When the voltage VOUT exceeds the value of the regulation voltage, the current on the reference branch 22 increases. The current is thus brought at input to the inverter 29 via the current mirror 30. The signal for enabling charge transfer VON/OFF switches and governs the charge pump into the inhibited operating condition. When the voltage VOUT on the output of the charge pump 2 drops below the regulation voltage value, normal operation of the charge pump 2 resumes.
The regulator circuit 20, however, presents some disadvantages. In the first place, it is evident how the regulation voltage depends upon the characteristics and upon the number of Zener diodes used and is not regulatable during use. In particular, the difficulty and costs of production, as likewise the effect of the inter-die process dispersions, increase with the number of Zener diodes used. Furthermore, since the regulation voltage is proportional to the number of Zener diodes used, it proves problematical to implement a fine adjustment to compensate for any possible inter-die dispersions of electronic circuits, such as for example, memory cells (not illustrated) to which the output of the charge pump 2 may be connected in use.